High Pressure Synthesis of Low Dimensional Pnictogen Chalco-Halides Based on (Sb,Bi)(S,Se)(Br,I) for Emerging Energy Applications

There is an increasing interest in the development of novel van der Waals materials with low dimensionality and with exotic properties, for emerging energy applications such as photocatalysis, photovoltaic and thermoelectric devices. Among them, pnictogen chalco-halides with low dimensional morphology (commonly quasi-one-dimensional nanocolumnar structures) are attracting a lot of attention due to the unusual combination of bandgaps suited for different energy applications, excellent stability, and possible high tolerance to defects. Despite these clear advantages, it is very complex to synthesize this type of materials containing one halogen and one chalcogen in the structure, even if there are clear evidence that these materials can be synthesized at relatively low temperature (< 400 C). In this work, we will present first a detailed DFT modelling of the structure, stability and optical properties of pnictogen chalco-halides based on (Sb,Bi)(S,Se)(Br,I). Theoretical calculations confirm the high stability of all possible ternary compounds, with band structures and bandgaps very well suited for different energy applications. Then, we will present an innovative synthesis route for complex pnictogen chalco-halide compounds based on a sequential process, starting with the co-evaporation of chalcogen thin films, followed by a high-pressure reactive annealing (from 2 atm up to 20 atm) under halogen atmosphere. Using this innovative approach, the complete set of ternary van der Waals (Sb,Bi)(S,Se)(Br,I) chalco-halide semiconductors are synthesized, including: SbSBr, SbSI, BiSBr, BiSI, SbSeBr, SbSeI, BiSeBr, and BiSeI. It will be also demonstrated for the 8 compounds, that high purity phase can be synthesized through a thin film-to-nanocolumns topological transformation. All the possible ternary compounds exhibit orthorhombic Pnma structure with quasi-one-dimensional nanocolumnar morphology; which thicknesses, heights, and densities can be tuned by changing the temperature and pressure during the synthesis process. The bandgap of the materials ranges from 1,25 eV for BiSeI up to 1.93 eV for SbSBr, covering a wide range of energy applications and correlating very well with the DFT calculations. A complete comparative analysis of the morphology, structure, optic, and electric properties of the 8 compounds will be presented. As examples, two possible applications of these (Sb,Bi)(S,Se)(Br,I) low dimensional structures will be proposed. In one hand, first solar cell prototypes will be shown, with very encouraging open circuit voltages around 600 mV, and conversion efficiencies exceeding 1% level in a very short period. In a second hand, some of these compounds were tested as visible light sensitized of TiO₂photocatalytic activity for hydrogen production, demonstrating an impressive hydrogen evolution increase of up to 90% under white light illumination. Finally, a comparative analysis and possible future applications of these novel low dimensional materials will be presented, and a perspective for their future accelerated development will be proposed.